No-take marine reserves, which restrict fishing in a certain area, give struggling fish populations a reprieve and boost biodiversity. But how can managers tell if people are following the rules, especially when the reserve covers vast and remote stretches of ocean?

A research team decided to investigate whether the presence of lost fishing lines could serve as a reliable indicator of illegal fishing. Gear often snags on coral reefs, and once abandoned, it can remain entangled for decades.

The researchers conducted their study around three island groups in the Great Barrier Reef Marine Park, Australia. Some of the reefs surrounding these islands were designated as no-take marine reserves (NTMRs) in 1987, and new NTMRs were added in 2004. The team dove about four to nine meters deep at 94 sites, half in NTMRs and half in open fishing areas, and looked for abandoned fishing lines.

At 10 of the sites, volunteer divers cleaned the area of lines, hooks, and other gear. Again, half the sites were in NTMRs and half were not. Two and a half years later, researchers surveyed those sites for the re-appearance of fishing lines.

The density of lost fishing lines in no-take marine reserves (established in 1987 or 2004) and open fishing areas at three island groups in the Great Barrier Reef Marine Park.

At two of the island groups, fishing line density was lowest at sites designated as NTMRs for more than two decades, the team reports in PLOS ONE. But there wasn’t much difference between the line density in recently-created NTMRs and open fishing areas. Much of the garbage may have accumulated before the NTMR was established, making it hard to figure out whether the amount of discarded gear reflected illegal activity.

The sites that were cleaned, however, provided more clues about compliance levels. Two sites in no-take marine reserves “were clear hotspots for illegal fishing,” the team writes. Of the 167 lines removed from NTMR sites, 150 came from those hotspots. When the researchers later re-surveyed the cleaned sites, they found that the amount of newly-deposited fishing line at the NTMR sites was about one-third the amount at open fishing sites, suggesting that levels of illegal fishing were higher than expected. — Roberta Kwok | 8 January 2015